docs: remove broken opentracing example for now

[ciarams87]

Proposed changes

This commit remove the broken Opentracing example. The example will be re-added when it can be fixed and verified as working correctly.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto master
• I will ensure my PR is targeting the master branch and pulling from my branch from my own fork

[pleshakov]

@ciarams87 there are few references from the docs. could you remove them?

./docs-web/configuration/global-configuration/configmap-resource.md:408:     - `Support for OpenTracing <https://github.com/nginxinc/kubernetes-ingress/blob/master/examples/opentracing/README.md>`_.
./docs-web/configuration/global-configuration/configmap-resource.md:412:     - `Support for OpenTracing <https://github.com/nginxinc/kubernetes-ingress/blob/master/examples/opentracing/README.md>`_.
./docs-web/configuration/global-configuration/configmap-resource.md:416:     - `Support for OpenTracing <https://github.com/nginxinc/kubernetes-ingress/blob/master/examples/opentracing/README.md>`_.
./docs-web/third-party-modules/opentracing.md:5:This document explains how to use OpenTracing with the Ingress Controller. Additionally, we have an [example](https://github.com/nginxinc/kubernetes-ingress/tree/master/examples/opentracing) on how to enable OpenTracing for a simple web application using Jaeger as a tracer.

[pleshakov]

👍

this is needed to be cherry picked to the release branch, right?